1. Field of the Invention
The invention relates to reforming degraded areas in ductile materials, in particular by melting a localized area to a predetermined depth, re-forming the localized area by cooling it, and advancing the localized melting and cooling through the degraded area to restore it to an integrally continuous form. The melting preferably involves scanning over a surface along successive adjacent lines on the inside surface of a tube, to progressively melt and fuse over defects at least to a depth sufficient to restore the tube to a serviceable condition for effecting heat exchange, pressure confinement or the like. The invention is particularly applicable to fusing service-induced stress and corrosion defects in coolant circuit tubes of pressurized water nuclear reactors. Localized melting is preferably accomplished by welding, especially with a laser weld head. An alloying material can be included during welding, the alloying material being supplied as a powder, gas or self-supporting tube and consumed in the process.
2. Prior Art
Heat transfer tubes in the steam generator systems of nuclear power plants are subject to degradation over time of the primary pressure barrier in zones of high mechanical stress. The pressure and temperature of coolant in the coolant circuit can be substantial. Static pressure due to the coolant may be on the order of 100 bar (2,200 psi) in the coolant circuit and the coolant temperature can be 600.degree. F. when the plant is operational. The thermal and mechanical stresses applied to the tubes tend to degrade the tubes in the regions of highest stress.
One area of concern is the tubing in heat transfer devices. In a reactor system, heated water from a primary coolant circuit is passed through heat exchanger tubes having a second flow of water passing outside the tubes. Should the barrier defined by a tube deteriorate, the secondary coolant may become contaminated with radiation from the primary coolant. It is expensive to replace the heat exchangers, and methods have evolved for repairing degraded steam generator heat exchanger tubes in various situations. However, the inaccessibility of the heat exchanger tubes as well as the radioactive environment surrounding the heat exchanger tubes present difficult problems.
A steam generator heat exchanger generally has a number of substantially parallel-flow tubes coupled between inlet and outlet manifolds. Whereas there are parallel alternative flow paths, one method of dealing with a deteriorated steam generator tube is to simply plug lit to prevent leakage. Plugs are inserted upstream and downstream of the plugged zone, isolating the zone of deterioration from the primary side coolant, and thus preventing leakage.
Plugging an affected tube reduces the active heat exchange surface area and results in a reduction in steam generator performance. Designers build an extra margin into the design of heat exchangers of this type, in recognition that it may become necessary in the future to plug some of the heat exchanger tubes without replacing the entire unit. If carried to an extreme in which the number of plugs installed exceeds the plugging margin provided according to the steam generator design, the steam generator must be de-rated because the rated heat generation capacity exceeds the available heat transfer capacity of the heat exchanger.
An alternative repair known as sleeving involves isolating only the surface of the tube in the area of deterioration while allowing primary coolant to flow in the tube, i.e., without removing the entire tube from service. Sleeving involves fitting an undersized length of tubing (the sleeve) into the affected tube, and attaching the sleeve upstream and downstream of the degraded zone to the inner walls of the tube, preferably sealing the sleeve to the tube to prevent leakage between them. The sleeve forms a seal and restores the integrity of the pressure boundary. The sleeve may be attached to the walls of the tube by mechanical means, such as by forming complementary bulges in the sleeve and the tube after positioning the sleeve to lap the area to be repaired. For a hermetic leak-tight seal the sleeve can be welded to the tubing, for example using gas tungsten arc or laser welding. Alternatively, the sleeve and tube can be attached by brazing. In either case, the sleeve is joined to the tube along the ends of the sleeve, typically along a circular line at each end of the sleeve.
The sleeve does not completely occlude the tube like a plug, but there are penalties. Thermal transfer declines due to flow reduction resulting from the reduced internal diameter of the tube and the discontinuity defined by the sleeve. In an extensive sleeving program, or in a generator that already has a large number of plucks, sleeving may not be practical. The thermal transfer characteristics of the heat exchanger tube are adversely affected by a sleeve. The sleeve thickens the overall tube wail. Any area of relatively lower thermally conductive contact between the sleeve and the original tube, such as a gap or corroded zone, forms an insulating zone, reducing the heat transfer efficiency of the system. The effect of a single sleeve may not be large, but large scale sleeving, especially in the support plate regions, can significantly reduce the efficiency of the steam generator.
The attachment zone of the sleeve, depending on the specific method of attachment to the tube, produces a local stress concentration in the original parent tube. The zones of attachment are at increased risk of further primary water attack with continued operation of the generator, requiring additional treatments or operations to reduce susceptibility to degradation at the attachment zones. Sleeves thus introduce additional process steps, down-time, and costs for materials and equipment.
Sleeving for a steam generator plant of this type must be made to specific requirements of the ASME Code. Because applications vary, the length and configuration of the sleeves needed varies as well, making it necessary to stock a large variety of sleeves to enable repairs at different areas.
Laser welding is one means to attach a supplemental sleeve inside a tube. U.S. Pat. No. 4,694,136--Kasner et al discloses welding the ends of a sleeve to the inner walls of a heat exchanger tube in this manner. After placing the sleeve inside the tube and mechanically forming complementary annular bulges in the sleeve and the tube to fix them against displacement, a 500 to 700 watt laser beam is applied to the inner surface of the sleeve, either in successive closely spaced lines or in a helical continuous line. The area melted by the laser is about 0.24 inches in width (0.61 cm) and extends completely through the material of the sleeve, and about 0.025 inches (0.064 cm) deep into the tube, or about halfway through the tube wall. The melted material of the sleeve and tube are mingled and fused around the circumference at the point of attachment, forming a hermetic connection of the sleeve and the tube.
According to Japanese Patent Publication 2-199,397, dated Aug. 7, 1990, it is known to heat-treat degraded areas of a tube along the inner surface as a means to reduce the later occurrence of cracking caused by tensile stress and corrosion. The incidence of stress-corrosion cracking in austenitic stainless steel is particularly increased if the steel has been heated to a temperature between 550.degree. and 800.degree. C. (about 1,000.degree. to 1,500.degree. F.). The phenomenon of increased cracking within this temperature range, known as sensitization, occurs due to precipitation of carbides from solution with the iron, especially along interstices between fine granular bodies. Typically, in the production of steel, care is taken to cool the steel quickly through the sensitization temperature range, to minimize the degradation of structural integrity caused by sensitization. According to said Japanese Patent Publication 2-199,397, heat treating is accomplished using a YAG laser beam to produce a temperature rise on the inner surface that is limited to below the temperature of sensitization. The laser beam is focused at a localized internal surface of the pipe and moved over a predetermined axial length at a sufficient rate to limit the temperature rise. A chromium or titanium powder can be applied to the internal surface prior to heat treating, for improving the stress-corrosion characteristics of the tube as a part of the heat treating process. The heat treatment is described as thereby assisting in either preventing stress-corrosion cracks or repairing stress-corrosion cracks after they have occurred.
Plugging and sleeving deal with problems in the structure of a tube by isolating the degraded portion of the tube from the primary coolant. Heat treating prior to the occurrence of cracks may be helpful, but heat treating after cracks have occurred requires the application of additional material, and therefore resembles sleeving. It would be desirable to reconstitute the tube rather than to patch over or isolate it to deal with a deteriorated zone. Reconstituting the tube in situ by actually melting and then solidifying the tube material would preclude the need for add-on structures adversely affecting the flow characteristics of the tube or the plant. An additional material can be employed for alloying with the material of the original tube, or the tube can simply be reformed from its original metal.
Whereas melting a metal allows the metal to flow, it would appear to be impossible to reform a tube in this manner without providing some form of mold for support. However, by melting only a small localized area at any one time, e.g., by laser welding, proceeding along the length of the tube to be repaired in a generally helical or axial scanning pattern, and/or along parallel lines or the like, it is possible according to the invention to reform the tube incrementally and to fuse over stress-corrosion cracks. Moreover, by melting a small area at a time using a focused laser, the melted metal is retained in place by the surrounding solid metal. As the point of welding passes, the thermal sink provided by the surrounding metal quickly cools the solidified metal through the sensitization range.
It is an object of the invention to repair a tube having a deteriorated area, especially a heat exchanger tube in the heat exchanger of a nuclear steam generator plant, by melting a depth of the inner surface of the tube using welding technology, thereby fusing over cracks and rendering the tube material once again continuous over the zone of deterioration, and to a sufficient depth to return the tube to serviceable condition.
It is a further object of the invention to repair a pressure vessel tube without substantially decreasing the internal diameter of the tube or increasing the external diameter, by melting and reforming the tube material at a localized small area, and scanning the area of localized melting to proceed over the tube surface in adjacent or overlapping lines.
It is another object of the invention to provide a means to repair a tube conveniently, which improves over results obtainable either by plugging the tube, by adding a supplementary support/sealing sleeve, or by heat treating the tube in the presence of a supplemental material. In particular it is an object to melt and reform the tube in incremental lines so that cracks are fused over and the heat exchange capacity of the repaired tube is at least as good as that of the original tube.
It is also an object of the invention to employ a tracking optical welding technique using a high powered laser, for surface welding at least a depth at the inside of a tube along a progressive overlapping spiral path, at a sufficiently slow rate of advance and a sufficiently high power level, to melt the tube at an isolated area that quickly cools after the welding point passes.
It is another object of the invention to improve the surface of a pressure vessel tube by surface welding, optionally in the presence of an alloying material which modifies the characteristics of the original tube in the area of the repair. The alloying material can be a welding powder, gas or consumable sleeve disposed in the degraded area prior to or concurrently with welding.
These and other aspects of the invention are met in a method for repairing a wall of a pressure vessel tube having a degraded area by melting a localized area to a predetermined depth, re-forming the localized area by cooling it, and advancing the localized melting and cooling through the degraded area to restore it to an integrally continuous form. The repair is effected by positioning in the tube a high powered laser welding head with the beam energy focused at a limited area or spot on the internal wall of the tube. The welding head is relatively moved in the tube at a sufficient rate to weld along a line on the internal wall for melting the tube in the degraded area at least to a depth equal to a part of the thickness of the tube wall. The depth of melting is sufficient to restore the tube to serviceable condition by reforming a functionally sufficient thickness of tube, with regard to the particular purpose of the tube. The point at which the tube has been melted by the focused energy cools following passage of the welding head. As the point of application of the welding head advances along a line, the line is likewise advanced laterally. Localized melting and cooling of the tube material continues, line by adjacent or overlapping line, to encompass the degraded area. By successively melting, and optionally applying one or more alloying materials such as a powder, aerosol or insert, using materials known in the art, linear sections of the wall, stress-corrosion cracks and similar defects are fused over, repairing the degraded area. The optional additional welding material can be employed for improving the characteristics of the tube as compared to the original tube. Preferably, the lateral advance is less than the width of localized melting, causing the lines to overlap and continuously re-form the degraded surface. Although the entire surface of the tube can be melted and reformed in this manner, the localization of the melting at any one time is such that the metal does not substantially flow and the metal cools quickly following passage of the weld head, to below the temperature of sensitization. The advance of the welding line and the lateral displacement can be stepwise or continuous, and can be oriented axially or radially.
The realization of these objects will be appreciated from the following discussion of particular exemplary embodiments of the invention. However it should also be appreciated that the invention is capable of variation from the examples, in accordance with the scope of the invention as claimed.